High-frequency system



Dec. 3, 1940.

- FIGZI -O O O O I O W. E. KIRKPATRICK HIGH- FREQUENCY SYSTEM Filed NOV. 11, 1937 GRID ANODE WORK DONE 0N ELECTRONS wank 0on5 01v ELECTRON! J, K g

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I INVENTOR WE. K/R/(PATR/CK ATTORNEY Patented Dec. 3, 1940 PATENT OFFICE HIGH-FREQUENCY SYSTEM William E. Kirkpatrick,

New York, N. ,Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y. a. corporation of New York Application November 11, 1937, Serial No. 174,074

13 Claims.

This'invention relates to a high-frequency system, and more particularly, to a method and means for controlling the extent of input or active grid loss in such a system employing an electron discharge device.

It has been found, in the operation of an ultrahi 'gh-frequencyv system employing an electron discharge device comprising a cathode, an anode and a control grid, that, as the frequency or frequencies involved become higher and higher, for example, of the order of 100 megacycles, there is an increasing and a marked input or active grid loss, resulting from a decreasing value of a shunting resistance effectively existing across the input, or in other words, a decreasing value of the input impedance of the device.

An object of this invention is to control the input or active grid loss in an ultra-high-frequency system and either to decrease it, to annul it or to overcompensate for it.

In accordance with this invention, this object is realized, by placing an auxiliary electrode or grid between" the control grid and anode of the electron discharge device, the auxiliary electrode 5 being capable of secondary electron emission and which follows hereinafter, taken in conjunction with the appended drawing, wherein:

Figs. 1 to 5 are diagrams to be used in explaining the invention; and

Figs. 6 and 7 show a high-frequency amplifying system and a high-frequency oscillator system, respectively, embodying the means and method of the invention.

Fig. 1 shows the-geometrical disposition of the elements of an electron discharge device comprising a cathode, an anode, and a control grid. Let it be assumed that the anode is tied down to the cathode through a large condenser effective at all the frequencies of interest; that the anode is maintained at a fixed potential positive withrespectfto the cathode, and the grid at a fixed negative potential with respect to the oathode; that a small alternating potential is. impressed between grid and cathode, the frequency o fthis alternating potential being so low that an electron leaving the cathode region at any point of the cycle, reaches the anode before the value of the alternating has'changed.

potential on the grid Fig. 2 shows the work done on an electron performing under these conditions on its movement from cathode to anode, released at the instant of maximum positive alternating grid potential. The reversal of sign of the work done 5 on the electron at the instant it passes through the grid plane comes about because the force onthe electron caused by thewalternating potential is always directed toward the grid while the direction of motion of the electron is always 10 towardtheanode. For this low frequency case, the area Ais equal to the area B, since the electron rises and falls through an equal potential range.

When the frequency becomes sufficiently high 15 so that the alternating grid potential is no longer constant while an electron passes from cathode to anode, a difierent situation exists. Consider again the case of an electron released at the instant of maximum positive alternating grid 0 potential. Bythe time the electron has reached the grid plane, the force due to the grid potential has decreased since the alternating grid potential is no longer at its maximum positive value which existed when the electron was in the cathode region, and the area A under the solid line of Fig. 3 indicates the amount of positive work done on the electron.' During the passage of the electron from the grid to the anode plane, the alternating grid potential continues to decrease; the area B above the solid line of Fig. 3, indicates the amount of negative work done on the electron in this region. The dotted lines show the boundaries of the area A and areaB of Fig. 2. 1 Because the alternating potential force has been steadily decreasing while the electron passed from the cathode to the anode, the amount of negative work done on theelectronis less than the amount of positive work, and the amount of total work done on the electron, therefore, is i positive. This positive work must be supplied a at the expense .of the alternating current energy.

It constitutes the input or active grid loss, the existence of which has been recognized only since the recent advent of ultra-high-frequency. As 4 1 the input frequency increases, the effective input impedance decreases, and as the frequency employed becomes ofthe order of magacycles, for

example, megacycles, the input impedance becomes'so low as to seriously affect the amplifi- 5d cation or oscillating characteristics of an ultrahigh-frequency system. It is desirable and necessary, therefore, that, in the case of a high-fre quency; amplifier, this input or active gridloss be effectively reduced or annulled, andin the'g'g case of a high-frequency oscillator, be effective- 1y reduced, annulled or over-compensated. An-

other way of considering it, is thatv effectively the input impedance must be kept as high as possible, and many times larger than the impedance of the input circuit.

If we consider the work done on an electron leaving at the instant of maximum negative alternating potential, the reverseof the above effeet is obtained and the total amount. of Work done on an electron released at this instant is negative. This negative work yields up energy to the alternating current cycle andltends to build up the alternating currentfFewefelecof the effect of the electrons released at the maximum positive alternating grid potential is necessary for this'type of, analysis since the'over-all effect of the 'oper'ati'on'of theidevice or tube is the same.' v "I f' If it were possible to increase suddenly the number of electrons flowing to'the anode in the grid-anodevreg'ion, these electrons would come under the influence of the anode lines of force and join the electrons flowingfrom the cathode to the anode. .These added electrons would yield up their negative 'or positive: energy to the systern along with those that originated at the cathode. Ifthere is considered only the short time interval in which electrons released from the cathode region at the moment of maximum positive alternating grid potential flow to and reach the. anode, Fig. 4 may be drawn. In that fig1ire, the cross-hatched area C represents added negative Work done because of electrons added at the plane of the screen grid, XX a manner to be disclosed presentlyg. Area- A. is the same as that of Fig. 3; butarea Bof Fig. 4 is less than area B of Fig. 3 because some oi 5Q theelectrons that originate at the cathode will be stopped atthe screen grid and will not reach the anode, and, hencacannot contributeto. the negative work. 7 The negative work yielded ;up by the added electrons more than, makes upfor the slight decrease resulting from the; interception of primary electrons,

In accordance with this invention by the:

sertion along the XX plane of Fig. 4 of a screen or auxiliary electrode treated so that oneelectron. impinging onits surface would release sev,-, eral electrons, i. e., a screen havingsecondary electron emissionproperties, this. effect may be attained. Fig. 5 shows the geometrical disposition of 5110 a, screen with reference to the cathode, anode and control gridof a triode. The a rs'v e e r e ld h m intai e a a positive potential with respect to the cathode, but at a lower value than that of the anode, this potential being to attract primary electronsto the screen. The screen cou'ld be lined up with 'the spaces in the control grid so that the primary electrons would be more certain to strike the screen. Thefsecondary electrons released from e' ra W i -l find thems e in t 7d field of theistronger force produced byithehigher" trons, however, are released, from the cathode region when the grid is in-its-negative alter'natanode potential and would be drawn to the anode, thus producing the increasein negative work indicated by C' in Fig. 4, which results in a reduction in the active grid loss.

The action of the screenin reducing active grid loss may be considered in greater detail. Let it be assumed thatthe screen grid is tied down to the cathode through a large condenser effective at all the frequencies of: interest. The condition of operation of the other electrodes is as noted hereinabove. Let thealternating impulse impressed between the grid and cathode be of high enough frequency to produce active grid loss in the tube. If one starts with zero direct potential applied between cathode and screen grid, and gradually increases the posi-.

tive direct potential of the screen, then the negnegative work evidenced by area 0 is made great enough, the active grid losscan be annulled or even overcompensated. Underv these conditions, the conventional amplifier, modulator, demodulator, oscillator or any other system depending ative work evidenced by area C of Fig. 4 will for its operation on an electron-discharge device or tube will be: more efiicient at ultra-highfrequencies when the secondary emission. screen electron. discharge device ill; comprises a 02th 1 ode l I, for example, of the. indirectly heated type,

electrode or screen grid 14 having secondary electron emission properties for the purpose already pointed out hereinabove. Atunedinput circuit l5 comprising the secondary t6 of an input transformer l1 and the shuntcondenser' I 8 are connected in series with grid biasing battery [9 in the grid-cathode circuit. A source 20 0f ultra-- high-frequency current to be amplified is connected across the primary winding 2| of the transformer I1. I The cathode-anode circuit includes anode battery 22 and-output network2'3 comprising an inductance-'24, a condenser 25,'an'd' a load resistance 26. The screen grid I4 is con nected' with the anode battery-so that it has an adiusted direct potential applied-to it that is less than that of the anode. ."Bygpa'ss j coir-1 densers 21, 2 8 and 29 are connected between the grid and cathode, cathode and screen grid, and

cathode and anode.

In operation, ultra-high-freduencies, which the cathode H to'the anode [3.] Because of the.

presence of the screen grid M, whose."potential has been adjusted to a valuefsuch' that 'sufficient. secondary electrons are added 'to' those. passing" from the cathode to the anode; the a ute grid: loss' is annulled or. compensated and the device functions asif there. were input or active grid loss.- The amplified signal is produced in the output network 23-, andmay be'p icked'off across theresistance 26.

Considered from another viewpoint, if the dea control grid [2, an anodel3, and'anauxiliary I vice were a triode, at ultraehighefrequencies its input impedance would become quite low in relation to theimpedance of theinput circuit. By utilizing the disclosed auxiliary electrode ar-, rangement, the input-impedance of the device is effectively increased to many times the value of the input circuit impedance, and for all practical purposes to an infinite value. There will be, therefore, no effective 10W, shunting resistance across the input circuit, and the active grid loss will be minimized to the extent determined by the direct potential on the auxiliary electrode.

An ultraehigh-frequency oscillator system is shown by Fig-7, elements of which correspond to those of Fig. 6 having like identifying characters. Inductance 30 and condenser 3| comprise a tunedcircuit across which oscillations will appear, when the combined area of B". and C (Fig.4) is made greater than that of A. Under such conditionaithe. effective resistance of the device is negative andoscillations will build up until the positive resistance caused by the circuit losses balances the negative resistance. The oscillations may be picked off across load resistance 32.

In the discussion with reference to Figs. 4 and 5, it was noted that the cathode, screen and anode were connected together for alternating current. In actual operation, the electrons that go to the screen produce no high-frequency energy in the anode circuit and so cause a reduction in the efficiency of the system. Enough electrons must go to the screen to produce the secondary emis sion required to reduce, annul or overcompensate for the active grid loss; therefore, the shielding action of the screen, consequent to its area and direct potential, should not be so large as to prevent an over-all gain in the number of electrons in the control grid-anode region that finally reach the anode over the number existing in the cathode-control grid space.

Although this invention has been disclosed with reference to certain specific embodiments, it is to be understood that it is not limited thereto, but only by the scope of appended claims.

What is claimed is:

1. In a high-frequency system, electron discharge means comprising a cathode, an anode and a control electrode, said electrode being biased to a potential negative with respect to said cathode, an input circuit and an output circuit for said means, said means having active grid loss, and an auxiliary electrode providing secondary electron emission for controlling the extent of the active grid loss, said auxiliary electrode being maintained at a potential with respect to said cathode less positive than said anode, and adding electrons to those flowing from said cathode to said anode.

2. In a high-frequency amplifier, an electron discharge device comprising a cathode, an anode, a control electrode and an auxiliary electrode, said anode and auxiliary electrode being maintained at potentials positive and said control electrode being biased to a potential negative with respect to said cathode, and an input circuit and an output circuit for said device, said auxiliary electrode providing secondary electron emission whereby the active grid loss that would be present in the absence of such secondary emission is substantially reduced, the potential of said auxiliary 1 3..Ihe method of controlling the active grid loss in electron discharge means comprising a cathode, an anode and a control electrode, that comprises providing secondary electron emission in said tube from a source, other than said cathode, anode and controlelectrode, to reduce, annul or overcompensate for such active grid loss.

4. In an ultra-high-frequency system, an electron discharge device comprising a cathode, an anode and a control grid, an input circuit, the input impedance of said device at ultra-high-frequencies being low because of active grid loss, and an auxiliary electrode in saidldevice for efiectively increasing said input impedance to a higher value at such frequencies by reducing the active grid loss, said auxiliary electrode having a secondary electron emission property.

5. An ultra-high-frequency system comprising an electron discharge device, an input circuit and an output circuit for said device, said device having a cathode, an anode, a control grid and a screen grid, said screen grid being treated to emit a number of secondary electrons for each electron flowing from said cathode to saidwanode .that strikes the screen grid, to add such secondary electrons to the primary electrons flowing to said anode, and being maintained at a potential such that the effective input impedance of the device is many times larger than the input circuit impedance.

6.,In electron discharge means comprising a cathode, an anode and an input control electrode and whose input impedance tends to become effectively smaller as the frequency of the input signal'thereto increases until at or above a particular frequency the discharge means becomes substantially inoperative, the method of compensating for this tendency which comprises providing secondary electron emission in said tube from a source other than said cathode, anode and input control electrode, to reduce, annul or overcompensate for such tendency.

7. In a high-frequency system, electron .discharge means comprising a cathode, an input control grid, an anode, and an auxiliary grid between said input grid and said anode, and having secondary electron emission property, and means for impressing across the cathode-input grid an input wave of a frequency of the order of tens of megacycles per second, the effect of active grid loss resulting from said high-frequency input being reduced by secondary emission from said auxiliary electrode.

8. In a high-frequency system, electron discharge means comprising a cathode, an input control grid, an anode, and an auxiliary grid having secondary electron emitting property, an input circuit comprising said cathode and input grid, an output circuit comprising said cathode and anode, and means for impressing an input wave of a frequency of the order of tens of megacycles per second on said input circuit only, active grid loss resulting from such high-frequency input being reduced by secondary electron emission from said auxiliary electrode.

9. In a high-frequency system, electron discharge means comprising a cathode, an input control grid, an anode, and an auxiliary grid having secondary electron emitting property, said auxiliary grid being positioned between said input grid and said anode, an input circuit comprising said cathode and input grid, an output circuit comprising said cathode and anode, and means for impressing an input wave of a highfrequency on said input circuit, active grid loss resulting from said high-frequency input being reduced by secondary electron emission from said auxiliary electrode. i

10. The: method of operating an electron discharge device comprising an electron emitting cathode, an anode, a grid, and a discharge controlling member to translate a high-frequency electric Wave, which consistsin applying a constant positive potential to said anode; applying to the grid a positive potential of such value that primary electrons from said cathode cause secondary electrons to be given mi from the grid, and such thatthe number of secondary electrons given off will decrease if the potential of the grid is decreased and increase if the grid potential is increased; varying the potential. of the discharge controlling member in accordance with the electric Wave; and producing an emission of secondary electrons from said grid to increase the number of electrons flowing to said anode during a complete cycle of the electric wave over the number that would flow to the anode. in the absence of said grid.

11. In a high-frequency system, an electron discharge device comprising a source of primary electrons, an input control grid biased to a potential negative with respect to the primary electron source, a second grid constituting a source of secondary electrons, and an anode, the second grid being located between the input grid and the anode, means for impressing across said primary electron source and said input grid an electric wave of such high frequency that the efiective input impedance of said device in. the absence of said second grid would be reduced to a value comparable to or less than that of the input circuit impedance, said second gridbeing maintained at such a positive potential with respect to said primary-electrons source and having such electron emissivity as to enable an over-allgain in the number of electrons in the input gridanode region that finally reach the anode over the number existing in. the cathode-input grid space.

12'. electron discharge system comprising a source of primary electrons, an anode, an input control grid between said electron source and anode, means impressing across said electronssource and grid an electric wave ofsuch high frequency that the alternating grid potential does not remain constant. while an electron passes-from said source to the anode, whereby the total work done on. the primary electrons for a complete cycleof the wave is positive, and means comprising a source of electrons to be added to the primary electrons flowing to said anode to increase the: amount of negative work done on the electrons in the system whereby the total work done on the electrons in the system for a complete cycle of the-inputwave is less positive than if said electron-adding means were omitted.

13;v An electron discharge system comprising a source of primary electrons, an anode, an input control grid biased to: a potential negative with respect. to: said electrons-source and positioned between said source and said: anode, means impressing across said source and said grid an electric wave of such high-frequency that the alternating grid potential does not remain constant while an electron passes from said source to said anode, whereby'the total work done on the primary electrons for a complete cycle of the input wave is positive, and means comprising a second grid having. a surface treated to emit secondary electrons when said primary electrons impinge. thereon and positioned between said. in-

put grid and said anode, said second grid being at a potential positive with respect to said: primary-electronssource but less than the positive potential of said anode, said secondary electrons being added. to the primary electrons passing to theanode to increase the amount of negative work done on the. electrons in said system so that the total work done on the electrons in said sys-- tem for a complete cycle of the input wave is less positive than if: said second grid were omitted.

WILLIAM. E. KIRKPATRICK. 

